A North Korean woman who posed as a defector and obtained information on the South's military through romantic affairs with its officers has been arrested for spying, South Korean prosecutors said.

News of the arrest came as ties between the Koreas have chilled after President Lee Myung-bak took office in February and angered the North by saying Seoul would stop what once had been a free flow of aid and tie handouts to progress the North makes in nuclear disarmament.

Prosecutors said in a news conference with local media that the suspect, 35, arrived in South Korea in 2001. She was arrested last month and charged with spying for the North. They also detained one of her lovers, a 26-year-old Army captain.

"We could only suspect that there might be spies mixed in with North Korean defectors during the reconciliatory mood of the past 10 years, and had no evidence," Kim Kyeong-su, a senior prosecutor at the Suwon District Prosecutors' Office, was quoted by Yonhap news agency as telling reporters.

"The suspicions have turned into reality," Kim reportedly said.

The woman is suspected of exchanging sexual favours for classified information on weapons systems and the locations of key military facilities, prosecutors said.

She then passed the information to North Korea, which is technically still at war with the South.

About 14,000 North Koreans have defected to the South after the 1950-1953 Korean War, with most of them coming in the past 10 years. Spy cases were commonplace for several decades after the war but have become rare in recent years.

Lawyers for the woman were not immediately available for comment.